Source synchronous I/O bus retimer

ABSTRACT

A device configured to recover and repeat source synchronous data. The device is configured to receive source synchronous data via a first interface and recover the received data utilizing a first clock signal which is generated to be approximately ninety degrees out of phase with the received clock signal. A second clock signal is generated to be in phase with the received source synchronous clock signal. The second clock signal is the utilized to select a newly generated clock signal and latched data for transmission in a source synchronous manner. The device is further configured to shift the phase of the generated first clock signal to be approximately ninety degrees out of phase with the received data signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to the field of digital communications and,more particularly, to the transmission of digital signals.

2. Description of the Related Art

The design of advanced digital communication systems requires carefulattention to the reliable transmission and receipt of signals in orderto ensure proper functioning. One popular design methodology is that ofsynchronous design. Generally speaking, synchronous designs utilize acentral clock which is distributed to various parts of the design. Whileutilizing a synchronous approach may simplify certain aspects of thedesign process, this approach is not without its problems. One suchproblem is that of clock skew. Because a central clock signal may bedistributed to various parts of a system via board traces, backplanes,interconnects, and the like, clock skew may exceed that which can betolerated by registers and other elements. Consequently, designtechniques to minimize clock skew must be utilized to ensure propersystem functioning. An additional consequence of using the synchronousdesign approach and distributing a clock signal throughout a system isthe inherent phase delay that may accumulate. In a design where higherfrequencies and performance are desired, these delays ultimately maycause the synchronous design approach to be untenable.

Source synchronous designs utilize a different approach than thesynchronous design approach in order to obtain higher performance.Source synchronous designs transmit both a clock signal and data from atransmitter to receiver. The receiver then uses the received clocksignal to recover the data. Because the clock signal and data aretransmitted from the transmitter to receiver in parallel, some of theproblems of a widely distributed clock signal are avoided. Consequently,higher frequencies may be more reliably maintained. Also, many sourcesynchronous designs may utilize both edges of a transmitted clock signalfor the transfer of data at a multiple of the transmitted clock rate. Inthis manner, even higher data throughputs may be achieved. Further,because both the clock signal and data are transmitted by a particularsource, longer trace lengths may be supported.

While source synchronous designs may provide certain advantages oversynchronous designs, they are not without their own problems. Forexample, the design must account for clock jitter due to noise. Inaddition, certain systems may require transmitting signals overrelatively long distances. Even with the greater lengths which may beachieved using source synchronous designs, jitter and other signaldegradation may limit the effective length of a transmission line.Further, signal degradation may also reduce the effective operatingfrequency which can be achieved.

What is desired is a method and mechanism for improving signaltransmitting signals in source synchronous designs.

SUMMARY OF THE INVENTION

Other embodiments and details of embodiments will be described in thedetailed description.

Generally speaking, a device is contemplated which is configured torepeat source synchronous data. In one embodiment, the device isconfigured to receive source synchronous data via a first interface,recover the received data and clock signal, and transmit sourcesynchronous signals corresponding to the received signals with improvedsignal integrity.

In one embodiment, the repeater device is configured to receive sourcesynchronous data via a first interface and recover the received data.The device includes circuitry which receives both a reference clocksignal and the received clock signal and generates a first clock signalfor latching the received data and a second clock signal. The firstclock signal is generated to be approximately ninety degrees out ofphase with the received data signal. The second clock signal isgenerated to be approximately ninety degrees out of phase with thereceived source synchronous clock signal. Additional circuitry isincluded which utilizes the reference clock signal and the second clocksignal to generate a third clock signal which is in phase with thesecond clock signal. Subsequent to using the first clock signal to latchreceived data, the third clock signal is then utilized to select a clocksignal and latched data corresponding to the received data and clocksignals for transmission in a source synchronous manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 illustrates one embodiment of a system employing a sourcesynchronous repeater.

FIG. 2 shows an example of signal degradation.

FIG. 3 illustrates one embodiment of repeater operation.

FIG. 4 illustrates one embodiment of a repeater.

FIG. 5 illustrates one embodiment of the operation of the repeater ofFIG. 4.

FIG. 6 illustrates one embodiment of the operation of the repeater ofFIG. 4.

FIG. 7 illustrates one embodiment of a repeater.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling with the spirit andscope of the present invention as defined be the appended claims.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of a source synchronous system 100configured to transmit signals. In the example shown, a source device102 is configured to convey signals to a destination device 104. Alsoshown in FIG. 1 is a retiming repeater chip 150 coupled between sourcedevice 102 and destination device 104. Retimer chip 150 includes inputport 110A which is coupled to receive source synchronous signals fromsource device 102 via bus 130. Retimer chip 150 also includes outputport 110B which is coupled to convey source synchronous signals todestination device 104 via bus 132. Also included in retimer chip 150 iscircuitry 120, and optional additional ports 110C and 110D.

In one embodiment, source device 102 and destination device 104 may bephysically separated by such a distance that signal degradationprecludes efficient signal transmission directly from source device 102to destination device 104. Therefore, rather than directly couplingsource device 102 to destination device 104, retimer chip 150 is coupledbetween the source 102 and destination 104 in order to improve thequality of the signal received by destination device 104. By improvingthe quality of the signal, higher transmission speeds and/or greatertransmission distances may be achieved by system 100. Generallyspeaking, circuitry 120 in retimer chip 150 is configured to removejitter in received signals, and amplify signals prior to retransmittingthem.

FIG. 2 illustrates one example of signal degradation which may occurduring transmission. Signal 200 illustrates an originally transmittedsignal. Originally transmitted signal 200 is transmitted with aparticular amplitude 210, and signal transitions occurring at particulartimes 220. Signal 201 illustrates one example of signal 200 aftertransmission and subsequent degradation. Due to noise, intersymbolinterference, and/or impedance, signal 201 has a lower amplitude 212than original signal 210. In addition, signal transitions 204A and 204Bmay not occur at times which correspond to the original signal 200. Forexample, transition 202 represents a transition which is timed accordingto the original signal 200. However, jitter may be present in the signal201 which causes a transition to occur earlier 204A or later 204B thandesired. Consequently, the overall integrity of signal 200 is degradedduring transmission as illustrated by signal 201.

Turning now to FIG. 3, one embodiment of a retiming repeater 150(alternately referred to as “retiming repeater”, “retimer”, or“repeater”) and its function is illustrated. In the embodiment shown,repeater 150 is configured to receive a source synchronous signal viaport 110A from device 507 and transmit a corresponding signal via port110B. Circuit 120 is configured to control acquisition and transmissionof the signals received and transmitted by repeater 150. Illustrated inFIG. 3 are a data line 503 and a clock line 505. A particular length 514of each line 503 and 505 is indicated. Device 507 transmits signals 502and 508 via lines 503 and 505 to repeater 150. Initially, signals 502and 508 have a relatively good degree of integrity with respect to theiroriginal amplitude and timing. However, after traveling along the lengthof lines 503 and 505, signal 504 which corresponds to the originalsignal 502 has reduced amplitude (is attenuated) and may include jitter.Similarly, signal 510 which corresponds to original signal 508 hasreduced amplitude and may include jitter. It is noted that thetransmitted data signal 502 and clock signal 508 need not operate at thesame frequency. Alternative embodiment may include source synchronoussignals wherein the clock signal is either a higher or lower frequencythan the corresponding data signal. For example, in one embodiment thetransmitted clock signal 508 operates at a fraction of frequency of thedata signal 502. In such an embodiment, the repeater 150 may beconfigured to multiply the received clock signal 508 in order to matchthe frequency of the received data signal 502. The multiplied clocksignal may then be utilized to capture the received data. Those skilledin the art will appreciate that numerous such alternatives are possible.

Generally speaking, if signal integrity becomes sufficiently degradedthrough attenuation or jitter, the system may cease to functionproperly. FIG. 3 illustrates the function of repeater 150 wherebydegraded source synchronous signals 504 and 510 are received, butcorresponding source synchronous signals 506 and 512 are repeated(transmitted) with improved signal integrity. In one embodiment,repeater 150 may improve and/or restore signal integrity by eliminatingcertain jitter components in received data and retransmitting signalswith increased amplitude. Consequently, because of the improved signalintegrity introduced by repeater 150, greater transmission lengthsand/or operating speeds may be achieved.

Repeating source synchronous signals may present especially difficultchallenges given the nature of the timing requirements for signals insuch systems. For example, because data and corresponding clock signalsare conveyed concurrently, jitter which affects the data and clocksignals to different degrees results in reduced ability to reliablylatch the corresponding data. Consequently, merely repeating receivedsignals in a somewhat blind fashion may only serve to increase jitter asthe signal is transmitted. Still further, as attenuation of signalsincreases, the ability of receivers to reliably identify the data and/orclock signals being transferred is also compromised.

FIG. 4 illustrates a portion of one embodiment of circuit 120. In theembodiment of FIG. 4, circuit 120 is configured to capture sourcesynchronous signals and repeat them in a reliable manner. Generallyspeaking, circuit 120 is configured to receive source synchronoussignals 452 and 454, and transmit corresponding source synchronoussignals 496 and 498 with reduced jitter and attenuation. It is notedthat signals 452 and 454 are illustrated as being received by receivers453 and 455, respectively, and transmitted as corresponding signals 455and 457. It is understood that delays may be introduced by receivers 453and 455. However, for ease of discussion, the signals received bylatches 456 and 458 may simply be referred to as input data 452. Also,the signals 454 and 457 may generally be referred to as the input clocksignal.

As shown, circuit 120 includes latches 456 and 458 which are configuredto receive input data 452. A first circuit 470, a delay locked loop(DLL-1) in the example shown, is coupled to receive input clock signal454 and output clock signals 459 and 809. Also shown in the embodimentis a system clock signal 802. System clock signal 802, which may begenerated locally or globally, is utilized by PLL 472 to generates aclock signal 805 which may generally serve as a reference clock signal.Clock signal 805 is coupled to DLL-2 471. Clock signal 459 is coupled tolatches 456 and 458 and is utilized to latch data into latches 456 and458. Clock signals 805 and 809 are coupled to DLL-2 471 which is furtherconfigured to convey clock signal 872 (DLL2_0). Latch outputs FFA 461and FFB 462 are input to multiplexor 492. Another multiplexor 494 isconfigured to convey either a logic high or a logic low signal. Clocksignal 872 is coupled as a select signal to both multiplexor 492 and494. Finally, multiplexor 492 is configured to convey output data 496,and multiplexor 494 is configured to convey output clock 498.

In one embodiment, PLL 472 is configured to generate clock signal 805with a frequency equal to that of the received clock signal 454. Systemclock signal 802 may be a clock signal operating with a frequency otherthan that of the received clock signal 454. In one embodiment, and forpurposes of discussion, the clock signal 805 generated by PLL 472 isapproximately twice the frequency of the system clock signal 802. Asclock signal 805 is generated locally it may represent a relativelyclean clock signal vis-à-vis the input clock signal 454 and may serve asthe basis for the ultimately transmitted output clock signal 498.Similarly, the data signals 461 and 462 which correspond to output data496 are also relatively clean (e.g., the data signals are restored totheir pre-attenuated levels and jitter is removed) vis-à-vis the inputdata signal 452.

As an overview, DLL-1 470 receives input clock signal 457 and conveyscorresponding clock signals 459 and clock signal 809. In one embodiment,DLL-1 470 is configured to generate clock signals 459 and 809 with a 90degree phase shift, or delay, vis-à-vis the received clock signal 457.In this manner, the clock signal 459 which is used for latching theinput data to latches 456 and 458 on alternate clock edges, may latchthe data when the data is more stable and may result in improved datalatching reliability. This latching of data will be discussed furtherbelow. In an alternative embodiment, clock signal 809 may be generatedwith a phase shift of other than 90 degrees vis-à-vis clock signal 457.

Multiplexor 492 is configured to receive signals FFA 461 and FFB 462which are output from latches 456 and 458, respectively. DLL-2 471 isconfigured to receive clock signal 809 and generate a correspondingclock signal 872 which may be approximately in phase with clock signal809. As already noted, in one embodiment, clock signal 809 may begenerated with an approximately 90 degree phase shift versus clocksignal 457. Consequently, clock signal 872 is derived from the “clean”clock signal 805 and may also be approximately in phase with the inputclock signal 809. Clock signal 872 is then utilized to select data foroutput from multiplexor 492 and select a signal for output frommultiplexor 494. Finally, data is transmitted from multiplexor 492concurrently with a clock signal 498 via multiplexor 494. Accordingly,source synchronous data 452 and clock 454 may be retransmitted as sourcesynchronous data 496 and clock 498 with jitter, noise, and other signaldegradation reduced or removed.

In the above example it is noted that clock signal 459 is generated tobe approximately 90 degrees out of phase with input data 455. As thereceived input clock signal 454 and input data 452 may have some phaseshifting with respect to each other, a clock signal with a 90 degreephase shift versus the input data may be more or less that 90 degreesphase shifted from the input clock signal 454. In one embodiment, DLL-1470 may be tuned during a power up test, or other procedure, to achievea desired phase shift between the generated clock signal 459 and theinput data 455. Consequently, the phase shift of clock signal 459 versusclock signal 457 may be more or less than 90 degrees. Those skilled inthe art will understand and appreciate that such tuning procedures arepossible. In addition, while it is possible to utilize phase shiftsother than 90 degrees, a 90 degree phase shift is used for purposes ofdiscussion herein. Still further, in an embodiment wherein additionalphase shifting is not performed on signal 459 as compared to signal 809,DLL-1 470 maybe configured to generate only one signal which correspondsto signals 459 and 809.

FIG. 5 illustrates one embodiment of a timing diagram corresponding tothe operation of the circuit 120 shown in FIG. 4. The timing diagram isnot intended to be precise, but rather is intended to be exemplary only.FIG. 5 shows a series of clock cycles during which source synchronousdata is both received and transmitted by circuit 120. In the exampleshown, a timeline 800 is shown which illustrates eleven half clockcycles of the received input clock 454. FIG. 5 shows a system clock 802and PLL clock 805 which is approximately twice the frequency of thesystem clock 802. Also illustrated in the diagram of FIG. 5 are inputdata 452 and input clock 454 as they are received at the input pads.Corresponding to the received data 452 and clock 454 are data rcv out455 and clock rcv out 457. Data rcv out 455 and clock rcv out 457represent the received data 452 and clock 454 taking into account delayswhich may be inherent in the receiving pads and buffers. In addition tothe above, signals DLL1-0 a 809 and DLL1_O 459 are shown, as well assignals 461 and 462 which are conveyed from latches 456 and 458,respectively. Signal DLL2_O 872 which is conveyed from DLL-2 471 isshown. Finally, output data 496 and output clock 498 are shown.

In the example shown in FIG. 5, at Time 0 input data 452 “A” is receivedwith a rising edge and first phase of input clock 454. Data “A” and thereceived clock 454 are shown delayed, due to, for example, pad or bufferdelays as mentioned above, as corresponding signals data 455 and clock457. DLL-1 470 receives both the input clock signal 457 and the clocksignal 805 generated by PLL 472. DLL-1 470 generates clock signals 459and 809 to be approximately 90 degrees out of phase with input clocksignal 457. Arrow 506 indicates where both signals 459 and 809 line upwith respect to phase. Also indicated in signal 459, and as will beillustrated in FIG. 6, is the phase margining or shifting which may beperformed on signal 459 to increase 504 or decrease 502 the phase shiftin order to bring it into an approximate 90 degree phase shift with datasignal 455. As already noted, this additional phase margining may bedetermined during boot or training sequence.

In the example shown, the received data 452 and clock 454 are in phasewith one another. Consequently, the 90 degree shift performed by DLL-1470 causes the rising and falling edges of clock signal 459 to occurapproximately in the center of the data “eye” of the input data signal455. In one embodiment, the input data 452 and/or input clock 454 signalmay be transmitted via differential signaling. Other embodiments mayutilize different signaling technologies. At Time 1, clock signal DLL1_O459 latches data “A” 455 into latch 456. At Time 2, clock signal 459latches data “B” 455 into latch 458. Latched values “A” 455 and “B” 455are conveyed to multiplexor 492 as values 461 and 462, respectively.Signal DLL2_O 872 which is output from DLL-2 471 is conveyed to bothmultiplexor 492 and 494. At Time 2, signal 872 falls to logic value “0”which conveys signal 461 (data “A”) from the multiplexor 492. Inaddition, at Time 2, signal 872 causes the logic value “1” to beconveyed from multiplexor 494. Accordingly, received data 452 and clocksignal 454 may be re-conveyed as source synchronous data 496 and clocksignal 498 with improved signal integrity as compared to the receivedsignals which may be degraded. For simplicity of discussion in theillustration, delays which may be inherent in latches 456-458,multiplexors 492-494, and drivers 496-498 have been ignored.

FIG. 6 provides an illustration of the operation of the embodiment shownin FIG. 4 wherein the received data and clock signals are not preciselyin phase with one another. In this example, input clock signal 454 isreceived approximately 90 degrees out of phase with respect to thecorresponding data signal 452. DLL_1 470 generates clock signal 809 witha 90 degree offset versus clock signal 457. However, if DLL-1 470 wereto merely generate latching clock signal 459 to that of clock signal457, with a strictly 90 degree offset, the edges of clock signal 459 maynot correspond to the center of the eye of corresponding data signal455. Therefore, as discussed above, DLL-1 470 is configured to shiftsignal 459 forward or backward as necessary to better align with thedata eye of signal 455. In this manner, not only is the data latchedwith a higher degree of reliability, but the jitter in the receivedsignals may be reduced or removed in the corresponding output signals 96and 498.

FIG. 7 illustrates an alternative embodiment of circuit 120. In theembodiment illustrated in FIG. 7, each of the components and signalsgenerally correspond to those discussed above and are numberedaccordingly. However, in this embodiment, each of the latched datasignals 461 and 462 are conveyed to further latches 456A and 458A,respectively. Use of latches such as 456A and 458A may be desired insome cases to account for any skew in signals 461 and 462 versus othersignals in circuit 120. Utilizing additional latches as shown may resultin an additional half cycle delay in the output signals 496 and 498.Also, in order to account for the additional latches, the signalsconveyed from multiplexor 494A as the output clock signal 498 have beenreversed. Numerous such alternative embodiments are possible and arecontemplated.

While the present invention has been described with reference toparticular embodiments, it will be understood that the embodiments areillustrative and that the invention scope is not so limited. Anyvariations, modifications, additions, and improvements to theembodiments described are possible. These variations, modifications,additions, and improvements may fall within the scope of the inventionsas detailed within the following claims.

1. A repeater device configured to repeat source synchronous data, saiddevice comprising: a first interface configured to receive the sourcesynchronous data comprising a first data signal and a first clocksignal; a second interface configured to transmit source synchronousdata; and circuitry coupled to said first interface, wherein saidcircuitry is configured to: utilize a reference clock signal and saidfirst clock signal to generate a second clock signal; utilize saidsecond clock signal to latch said first data signal; generate a thirdclock signal; and utilize said third clock signal to transmit in asource synchronous manner a data signal and a clock signal correspondingto said first data signal and the first clock signal via said secondinterface.
 2. The device of claim 1, wherein said circuitry isconfigured to generate said third clock signal out of phase with saidfirst clock signal.
 3. The device of claim 2, wherein said circuitrycomprises a first circuit configured to: receive said first clocksignal; receive said reference clock signal; and generate said secondclock signal to be approximately ninety degrees out of phase with saidfirst clock signal.
 4. The device of claim 3, wherein the first circuitis selected from the group consisting of: a delay locked loop, and aphase locked loop.
 5. The device of claim 3, wherein said first circuitis further configured to: generate a fourth clock signal approximatelyninety degrees out of phase with said first clock signal; and shift thephase of said generated second clock signal a first number of degrees tobe approximately ninety degrees out of phase with said first datasignal.
 6. The device of claim 5, further comprising a second circuitconfigured to: receive said reference clock signal; receive said fourthclock signal; and generate said third clock signal to be approximatelyin phase with said fourth clock signal.
 7. The device of claim 6,wherein said second circuit is selected from the group consisting of: adelay locked loop, and a phase locked loop.
 8. The device of claim 5,wherein the first circuit is trainable to determine said first number ofdegrees.
 9. A method for repeating source synchronous data, said methodcomprising: receiving said source synchronous data comprising a firstdata signal and a first clock signal; utilizing a reference clock signaland said first clock signal to generate a second clock signal; utilizingsaid second clock signal to latch said first data signal; generating athird clock signal; and utilizing said third clock signal to transmit ina source synchronous manner a data signal and a clock signalcorresponding to said first data signal and the first clock signal. 10.The method of claim 9, wherein said third clock signal is generated outof phase with said first clock signal.
 11. The method of claim 10,further comprising generating said second clock signal to beapproximately ninety degrees out of phase with said first clock signal.12. The method of claim 11, wherein said second clock signal isgenerated by a first circuit selected from the group consisting of: adelay locked loop, and a phase locked loop.
 13. The method of claim 11,further comprising: generating a fourth clock signal approximatelyninety degrees out of phase with said first clock signal; and shiftingthe phase of said generated second clock signal a first number ofdegrees to be approximately ninety degrees out of phase with said firstdata signal.
 14. The method of claim 13, further comprising: receivingsaid reference clock signal in a second circuit; receiving said fourthclock signal in the second circuit; and generating said third clocksignal to be approximately in phase with said fourth clock signal. 15.The method of claim 14, wherein said second circuit is selected from thegroup consisting of: a delay locked loop, and a phase locked loop. 16.The method of claim 13, further comprising training a first circuitwhich generates said second clock signal to determine said first numberof degrees.
 17. A source synchronous system comprising: a source deviceconfigured to convey source synchronous data comprising a first datasignal and a first clock signal; a repeater device coupled to saidsource device, wherein said repeater device comprises: a first interfaceconfigured to receive said source synchronous data; a second interfaceconfigured to transmit source synchronous data; and circuitry coupled tosaid first interface, wherein said circuitry is configured to: utilize areference clock signal and said first clock signal to generate a secondclock signal; utilize said second clock signal to latch said first datasignal; generate a third clock signal; and utilize said third clocksignal to transmit in a source synchronous manner a data signal and aclock signal corresponding to said first data signal and the first clocksignal via said second interface; and a destination device coupled tosaid repeater device, wherein said destination device is configured toreceive said transmitted source synchronous data from said repeaterdevice.
 18. The system of claim 17, wherein said circuitry is configuredto generate said third clock signal out of phase with said first clocksignal.
 19. The system of claim 18, wherein said circuitry comprises afirst circuit configured to: receive said first clock signal; receivesaid reference clock signal; and generate said second clock signal to beapproximately ninety degrees out of phase with said first clock signal.20. The system of claim 19, wherein said first circuit is furtherconfigured to: generate a fourth clock signal approximately ninetydegrees out of phase with said first clock signal; and shift the phaseof said generated second clock signal a first number of degrees to beapproximately ninety degrees out of phase with said first data signal.21. The system of claim 20, wherein said circuitry further comprising asecond circuit configured to: receive said reference clock signal;receive said fourth clock signal; and generate said third clock signalto be approximately in phase with said fourth clock signal; and whereinsaid circuitry is configured to utilize said third clock signal toselect for transmission in a source synchronous manner a data signal anda clock signal corresponding to said first data signal and the firstclock signal.